From what I've seen in the few weeks that I've owned a WP7 phone, nearly all "free" apps beyond the big guys like Evernote, Twitter, Facebook etc are ad-supported or to a lesser extent, demo quality. However, with the full version paid apps you can try them out before committing to purchase. I think that's a very nice alternative to the Android and iOS way of doing things, where (if there's no "lite" version) you must buy the app and if it sucks, you have to petition to get your money back.

As such, I've only seen a relative few apps with separate demo or trial versions. Microsoft's "try before you buy" model for paid apps makes that redundant and unnecessary. It appears the few app makers who do have such redundant versions are just porting from their already established Android or iOS offerings, where two versions of the same app are the accepted norm.